<stránka

	xmlns="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/strana"

	xmlns:m="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/makro">

	<nadpis>Aggregating data with Scheme</nadpis>

	<perex>counting records and computing sum</perex>

	<m:pořadí-příkladu>01700</m:pořadí-příkladu>

	<text xmlns="http://www.w3.org/1999/xhtml">

		<p>

			In <code>relpipe-tr-scheme</code> we can generate new records – not only modify records from the input.

			There is <code>--has-more-records</code> option which – if evaluated as true – says: „read one more record from the Scheme context and call me again“.

			We can also suppress all original records by <code>--where '#f'</code>.

			And we can also change the structure of the relation (see previous examples).

			Thus we can iterate through a relation but completely replace its structure and content.

		</p>

		<p>

			What it is good for? We can do aggregations – we can count records, compute sum, maximum, minimum or average value etc.

		</p>

		<m:pre jazyk="bash" src="examples/guile-file-count-size-sum.sh"/>

		<p>Usage example:</p>

		<m:pre jazyk="text"><![CDATA[$ ./guile-file-count-size-sum.sh /usr/share/icons/oxygen/

filesystem:

 ╭─────────────────┬───────────────╮

 │ count (integer) │ sum (integer) │

 ├─────────────────┼───────────────┤

 │            6260 │      31091700 │

 ╰─────────────────┴───────────────╯

Record count: 1]]></m:pre>

		<p>

			In SQL, the same result can be achieved by:

		</p>

		<m:pre jazyk="sql"><![CDATA[SELECT

	count(*) AS count,

	sum(size) AS sum

FROM filesystem;]]></m:pre>

		<p>

			This should be possible with <code>relpipe-tr-sql</code> in later versions.

			SQL is much more declarative and for many cases a better tool.

			In SQL we describe „how the result should look like“ instead of „how the result should be produced step by step“.			

		</p>

		<p>

			One day, there might also be a translator that parses SQL code and generates Scheme code,

			so we could have advantages of both worlds

			a) concise and declarative syntax of SQL and 

			b) streaming – which means no need for putting all the data in the RAM or on the disk.

		</p>

	</text>

</stránka>